My first thought was to just pipe the output from your system command to your perl code, something like:

    some_external_excutable_file $input | perl_prog.pl
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You can also run the system command from within your perl code. Take a look at open. About a third of the way down, examples like

    open(ARTICLE, "-|", "caesar <$article") ...
    open(ARTICLE, "caesar <$article |") ...
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are probably close to the type of code you want.

I'm not sure what you mean by "Is there any default variable for accomplishing it?". I may have missed your meaning: all the default variables are documented in perlvar, if that's of any help.